Telescopic jibs are used for cranes wherein the jib must be extended for use and retracted for other purposes, such as transport: Thus, such jibs are normally used for vehicular cranes. The sections of such jibs are typically tubular so that the successive sections can nest within each other when retracted and telescope outwardly to extend the jib to a desired length.
Such telescopic jibs execute hoisting operations with the load at their front end. As a result, the jib is exposed to a bending force in two main axes. Viewing the jib in cross section along its longitudinal axis, each jib section, when loaded, is subject to tensile stress on the upper side of the jib while, on the lower side, compressive stresses occur. Due to lateral forces and eccentric loading, horizontal bending and torsion also occur.
Designers of such jibs are principally interested in optimally configuring the cross-section for jib parts loaded in this way. Such a cross-section is easiest to devise when the maximum stresses are the same in every direction and approximate the permissible stress. These requirements are satisfied for instance in the case of thin-walled circular tubes or in the case of a square trussed structure only when uniform forces occur in all directions. If a cross-section is loaded, for instance, more in the vertical direction than in the horizontal, then an optimum rounded cross-section becomes an ellipse and an optimum cornered cross-section becomes a rectangular trussed structure, the cross-sections in both cases being higher than they are wide to account for the imbalanced forces.
A telescopic jib generally as described above is known, for example, from EP 0 499 208 B1. The cross-section of this telescopic jib consists of an upper profile part having a semi-box shaped configuration and a lower profile part, configured as a rounded half shell, welded to the free legs of the former. Although such totally round lower profile parts have good load introduction and stability properties, they do not compete with rectangular trussed structures with respect to stiffness. It is often necessary to install additional members, such as welded stiffeners, to promote stability to counteract buckling or to construct the jib of material that is somewhat thicker which has a negative effect on the weight of the jib overall.
A jib profile for cranes and vehicle cranes is known from EP 0 668 238 A1 in which the two upper leg sections of the lower profile, welded to the lower legs of the upper profile, are configured as straight strips. The remainder of the lower profile part has a curved shell shape. It is also proposed in this document, as an alternative, to employ a straight strip portion at another point of the lower profile part. These straight strip portions produce cross-sectional kinks in the profile at their edges. Due to these kinks the loading properties of such a profile once again approach those of a rectangular trussed structure; i.e., the stiffness can be increased. However, the drawback in such profile designs is that, due to the straight strips employed, the load introduction and stability properties which are particularly advantageous for curved profiles become poorer. Additional stiffeners or thicker material gauges are again needed which disadvantageously increases the overall weight of the jib.
German Utility Model No. 94 02 692 describes a jib profile comprising a substantially semi-box shaped upper section and a rounded lower section connected to the upper section, in which the lower section has at least one planar or flat wall section. This shape is utilized in an attempt to produce both sufficient resistance to buckling and sufficient load resistance against bending. A planar plate segment (wall section) is thus inserted into the cross-section of the lower profile. A disadvantage of this configuration is that planar plate segments or wall sections in such profiles strained by bending and buckling are weak points precisely with respect to buckling resistance. A further disadvantage of the planar segments is that, in the force introduction area between the points of overlap between adjacent jib sections, the planar strips or plates segments are substantially less able than curved shells to absorb transverse forces. Therefore, they have to be strengthened, for example by stiffeners, to counteract buckling.
DE 43 44 795 A1 describes a jib cross-section whose lower profile part consists of nine flat strips with adjacent stripe arranged at an obtuse angle with respect to each other. These strips form the plate segments of the lower profile part. They are all configured as flat plate segments, which again have the disadvantages regarding resistance to buckling.
Furthermore, DE 200 04 016 U1 describes a telescopic jib in which the coupling portion and/or at least one telescopic length consist of profiles, each of which having a lower, round part and an upper, semi-box shaped part, whose facing legs are welded to each other. The upper profile part has the shape of an isosceles trapezium without the longer base part, such that the legs of the upper and lower profile parts abut each other forming an angle which is smaller than 180° on the inner sides of the profiles. The lower profile part is made of material having relatively increased thickness. In this way, it is intended that a better resistance to buckling is achieved. For this purpose, however, the heavier lower profile part has to extend upwards far above the axis of the moment of inertia of the cross section, or the neutral zone, of the jib. Increasing the amount of material in the neutral zone is, however, not advantageous in a jib because it undesirably increases the weight of the jib itself.
Lastly, DE 196 24 312 C2 discloses a telescopic jib for a vehicular crane in which the upper profile part is semi-box shaped and the lower profile part consists of several shell segments adjacent to each other, each having an outwardly curved shape in the form of a circular arc. In this way, it is intended to combine the good load bearing and stability properties of curved profiles with the greater stiffness of a rectangular trussed structure, so that such a telescopic jib can be built particularly lightweight.
Despite the improvements achieved by the various shapes of the upper profile parts and lower profile parts of known jibs, there is still no optimum solution for extreme loads, such as in luffing jib operations, guyed or pre-tensioned systems, or when positioning a jib in an orientation approaching vertical. In such situations the tensile forces in the upper profile portion may be minimized, but large forces act along the main axis of the jib even while the load may be small, resulting in substantial lateral forces. The resulting lateral forces can be very large in these working positions, such that the jib may be in serious danger of buckling.